<p>Polygenic risk scores (PRSs) quantify genetic susceptibilities, yet ancestry imbalance in genome-wide association studies (GWASs) limits the accuracy of monoracial PRSs in non-European populations. Here, we perform a multiancestry GWAS meta-analysis for lung cancer (76,953 cases and 1,886,372 controls), identifying 87 conditionally independent genome-wide significant loci, including two unreported cytobands. We use a PRS construction method, PRS-CSx, to develop a multiancestry PRS (<InlineEquation ID="IEq1"><EquationSource Format="TEX">\({{{{\rm{PRS}}}}}_{{{{\rm{MA}}}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi mathvariant="normal">PRS</mi></mrow><mrow><mi mathvariant="normal">MA</mi></mrow></msub></math></EquationSource></InlineEquation>) which outperforms 32 published PRSs. To enhance predictive power, we construct a multitrait PRS (<InlineEquation ID="IEq2"><EquationSource Format="TEX">\({{{{\rm{PRS}}}}}_{{{{\rm{MT}}}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi mathvariant="normal">PRS</mi></mrow><mrow><mi mathvariant="normal">MT</mi></mrow></msub></math></EquationSource></InlineEquation>) using CatBoost, integrating 32 cross-trait PRSs across three ancestries. Combining <InlineEquation ID="IEq3"><EquationSource Format="TEX">\({{{{\rm{PRS}}}}}_{{{{\rm{MA}}}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi mathvariant="normal">PRS</mi></mrow><mrow><mi mathvariant="normal">MA</mi></mrow></msub></math></EquationSource></InlineEquation> and <InlineEquation ID="IEq4"><EquationSource Format="TEX">\({{{{\rm{PRS}}}}}_{{{{\rm{MT}}}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi mathvariant="normal">PRS</mi></mrow><mrow><mi mathvariant="normal">MT</mi></mrow></msub></math></EquationSource></InlineEquation>, we generate <InlineEquation ID="IEq5"><EquationSource Format="TEX">\({{{{\rm{PRS}}}}}_{{{{\rm{MAMT}}}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi mathvariant="normal">PRS</mi></mrow><mrow><mi mathvariant="normal">MAMT</mi></mrow></msub></math></EquationSource></InlineEquation> and validate it in independent cohorts (OncoArray, TRICL and All of Us). <InlineEquation ID="IEq6"><EquationSource Format="TEX">\({{{{\rm{PRS}}}}}_{{{{\rm{MAMT}}}}}\)</EquationSource><EquationSource Format="MATHML"><math><msub><mrow><mi mathvariant="normal">PRS</mi></mrow><mrow><mi mathvariant="normal">MAMT</mi></mrow></msub></math></EquationSource></InlineEquation> demonstrates superior discriminability in European, Asian, and African populations, improves risk stratification, and identifies approximately 10% additional lung cancer cases in the UK Biobank. Individuals with elevated PLCO_m2012 scores and high genetic risk exhibit a 12.64-fold higher cumulative risk than those with low scores and low genetic risk, supporting precision prevention strategies.</p>